Visual densitometer



Oct. 18, 1960 Filed Sept. 12, 1957 R. S. HUNTER VISUAL DENSITOMETER 2 Sheets-Sheet 1 o Barr-fev OFF l/ Il 9/ BW/ML.. m @www 'AA/w ,larme/vais Oct. 18, 1960 R. s. HUNTER VISUAL DENSITOMETER Filed Sept, l2, 1957 2 SheetsSheet 2 IN V EN TOR. E/CHHBD 5. HUN 7" E BY LUM.,

MM 'Ihm No United States Patent Oy VISUAL DENSITOMETER Richard S. Hunter, Falls Church, Va., assigner to the United States of America as represented by the Secretary ofthe Air Force Filed Sept. 12, 1957, Ser. N0. 683,674

1 Claim. (Cl. 88-14) This invention relates to the field of sensitometry and more particularly to a visual densitometer instrument.

As a background for providing a clear and a complete understanding of the present invention, as claimed, a densitometer of the visual type is disclosed in considerable detail in Fundamentals of Photography by Paul Boucher, published in 1955 by D. Van Nostrand Company, Inc., New York city, New York, at pages 94 to 97 and elsewhere; in Fundamentals of Photographic Theory by T. H. lames and George C. Higgins, published in 1948 by lohn Wiley & Son, Inc., New York city, New York, at pages 173 to 178, inclusive; in Photography by C. B. Neblette, published in 1939 by D. Van Nostrand Company, Inc., New York city, New York; and elsewhere. Patent Number 2,478,406 issued August 9, 1949, to Reginald T. Lamb presents a reasonably clear picture of representative problems in densitometry. L. P. Clerc in Photographic Theory and Practice published in 1954 by Henry Greenwood & Company, Ltd., London, England, at page 12, provides a table of relationships between density, opacity and transmission and, together with Boucher, expresses density units as the logarithm to the base of opacity, which is the reciprocal of transmission or transparency, which latter term is preferred by Boucher. Gamma is the ratio between any two densities on the straight line portion of the D log E curve, of a photographic material, and the difference in the logarithms of the corresponding exposures required to produce them, where D is density and E is exposure, as explained at page 193 in Neblette. Gamma is expressed graphically also as the tangent of the angle between the straight line portion of the characteristic curve and the horizontal axis. Gamma is an index to the amount of development received by the photographic material. A densitometer measures light intensities.

A brief summary of the invention follows, indicating its nature and substance, together with a statement of the object of the invention commensurate and consistent with the invention as claimed and also setting out the exact nature, the opera-tion and the essence of the inven- -t-ion complete with proportions and techniques that are necessary with its use. The punpose of the invention also is stipulated. T-he presentation is adequate `for any person who is skilled in the art and science to which the invention pertains to use it without involving extensive experimentation. The best mode of carrying out the invention is presented by the citing of a specific operative example, inclusive of the preparation and the use of at least one example of the invention.

The present invention is 'a densitometer of the comparison spot type. The densitometer contemplated hereby is a compact and la transportable article, provided with a switch for using either self-contained electrical energy supplied by batteries positioned within the article, or from a :line voltage source. The instrument is provided with a zero adjust control, for adjusting the position of a light source, and with a line bearing index arm which is transparent. The index arm is movable along a fixed density scale and along a fixed speed scale. The index arm also is movable along a gamma scale, which is movable with respect to the other two scales. The instrument is of the optical matching type and is used for indicating numerical values from a sensitometric test strip made on a photographic film by a sensitometer, from a photographic film or the like, which is removably -insertable in the instrument.

The objects `of the invention are the provisions of a new and an improved densitometer which is relatively small, which is conveniently handled and manipulated and from which readings, which are reproducibly accurate may be taken directly of density, speed and gamma values. The densitometer is adequately simple in both adjustment and operation for its satisfactory use by untrained personnel, who however must be capable of matching shades of grey.

in the accompanying drawings:

Fig. l is an external elevational view of the scales, the switch, and the zero adjust control knob bearing front panels of the densitometer which embodies the present invention;

Fig. 2 is an internal view of the densitometer shown in Fig. l with the front panel of the upper compartment removed and reversed upwardly and with the front panel of the lower compartment partly broken away to show batteries carried therein;

Fig. 3 is an enlarged light path diagram of the instrument shown in Figs. 1 and 2;

Fig. 4 is an enlarged side elevational view of the lamp with its mounting, as seen with the front `cover panel of the lower compartment of Fig. 2 removed and showing the :lower comparison lamp and the zero adjust mechanism mounted therein;

Fig. 5 is a plan view of an lapertured silvered mirror mounted in the upper compartment of the `device shown in Figs. l and 2; and

Fig` 6 is a series of observed views of the mirror shown in Fig. 5.

The instrument illustrated in the accompanying drawings consists of a first compartment 1 and a second compartment 2, joined together by an edge mounted hinge 3, to bring a pair of compartment walls 4 and 5 in overlapping relation with each other. A pair of laterally spaced apart, spring brass clips `6 are attached at their ends near the hinge 3, to the upper surface of the lower compartment upper wall 5, so that the spring bnass clips extend along the lateral edges thereof, with their unat- -tached ends remote from the hinge 3. The pair of spring brass clips 6 serve -to yieldingly retain la sensitometric strip, a photographic iilm, or the like, not shown, from which readings are to be taken, against the upper surface of the lower compartment upper wall S.

The optical system of the instrument, shown in Figs. 1 and 2 of the accompanying drawing and indica-ted diagrammatically in Fig. 3, is of the peephole type, with the peephole 10 extending through the upper wall 11 of the upper compartment of the instrument. The light beam observed through the peephole 10 is viewed as coming directly from a lower compartment light source 17, through pieces of opal glass 9 and S, the unfilled center 21 of the apertured silvered mirror 12 and a lens 22. Light yfrom a second light :source 16, Within the upper compartment of the instrument in Figs. 1 and 2, is reected by the mirror 12 to the peephole 10. The mirror 12 is mounted at about a 45 angle to the direction of the light beam from the light source 17 to the peephole 10, as indicated in Figs. 2 yand 3.

The upper compartment light source 16 is mounted in a lamp socket 40, which is attached on the inner side of the upper compartment front panel 18. Both of the lamps 16 and 17 are supplied electrical energy through a switch 15 from an alternating current fitting 14 and step-down transformer 13, or from a desired plurality of batteries 26, 26', etc. positioned within the lower compartment of the instrument over suitable electrical leads 41 or 41', respectively.

The lamp 17, in the lower compartment 2 of the instrument, is mounted in an adjustable socket 22. The adjustable socket 22 is connected by a. hinge 23 to the inner surface of the second compartment front panel or cover 19. The spring 24 loads the second lamp socket 22 against the lateral action of a finger operated screw 25 which is labelled zero adjust and is available on the outside of the lower compartment front panel 19 of the instrument. The zero adjust screw 25 threads through the panel 19 and contacts a spring thrust plate 20 which is secured to the lamp socket 22. The rotation of the Zero adjust screw 25 alters the space between the lower compartment light source lamp 17 and the lower compartment opal glass 9. Changes in the amount of illumination which falls on the opal glass 9, or changes in the intensity of the light energy which is incident to the opal glass 9 is accomplished by operation of the screw 25.

Light from the upper compartment lamp 16 is applied to the upper face of the silvered mirror 12, after it passes through a white plastic or glass block 30 at an adjustable inclination to the direction of the light beam. The white, translucent block 30 is attached at its upper end to a rotatable shaft 32 by a screw 43 or the like, such as at an angle of illustratively 15 with an index arm 33. The index arm 33 has its upper end also secured by a screw that threads into the end of the shaft 32, such that movement of the index arm 33 moves the block 30 correspondingly in both sense and amount.

The index arm 33 preferably is of colorless plastic, of which the colorless methacrylates or the like, are illustrative. The contact face of the index arm 33 has a hairline 34 extending longitudinally thereof, midway between its lateral edges for minimizing paralax.

The sweep of the index arm 33 is toward the left from a right-hand stop 35, which is bent upwardly from a surface sheet on the outer panel 18. The index arm 33 is journaled at its upper end on the shaft 32, so that its lower portion sweeps along two fixed arcuate indices, and outer scale calibrated in units of Density and an inner scale calibrated in units of "Speed Step Number in columns headed by 9, 11, 13 and 15. T he index arm 33 also sweeps along a movable sector 36 which bears a scale along its edge calibrated in units of Gamma The movable sector 36 is provided with an upturned ear 37 for a finger grip in accomplishing its displacement about the pin 32 on which it is journaled. The sector ear 37 rests against the stop 35 when the sector 36 is in its retracted position.

The visual densitometer contemplated hereby may be connected at its electrical fitting 14 with a source of electrical energy which passes from the fitting over leads 41 to the transformer 13. Output from the transformer 13 is applied to the A.C. side of the switch 15. The D.C. side of the switch 15 is supplied over suitable leads 41' from batteries 26, 26', etc. stored in the lower compartment Z of the device. The switch 15 has the three positions off," on and battery, at any one of which it may be set.

The densitometer upper compartment 1 may be turned on the hinge 3 sufficiently to permit the insertion, under the pair of spring clips 6 and over the lower compartment opal glass 9, of a film or other article of which the density, speed and gamma are to be determined. The densitometer upper compartment 1 is then returned to rest on the lower compartment 2, with the film retained therebetween.

The sensitometric readings for values for gamma, for speed and for the density of the film held beneath the 4 pair of clips 6, may then be taken directly from the scales of the instrument.

The zero setting of the densitometer is accomplished by placing the switch 15 at its A.C. or at its D.C. position, as preferred. The hairline 34 of the plastic arm 33 is placed at the 0 position on the Density scale. With no sensitometric strip between the opal glasses 8 and 9, the eye is applied to the peephole 10 and the zero adjust screw 25 is turned such that the oval or egg-shaped spot seen through the peephole 10, matches the illuminated eld around it, as is illustrated toward the right in Fig. 6 of the accompanying drawing.

Fog density of a clear unexposed area of a sensitometric strip is accomplished by inserting the strip beneath the pair of clips 6 and between the pair of opal glasses 3 and 9, or over the sample eld of the instrument. The plastic arm 33 is then moved from its density scale 0 position upwardly of the density scale until the spot 21 again matches the surrounding field. The reading under the hairline 34 of the plastic arrn 33 then is the fog density reading for the chosen sample area of the sensitometric strip.

Gamma determinations are made by placing a cleat area of the sensitometric strip over the sample field and zero set the densitometer by returning the plastic arm hairline 34 on 0 position of the density scale, look through the peephole 10 and adjust the zero adjust" screw 25 to match the spot with its field. Substitute for the clear area of the sensitometric strip, a medium gray step of the sensitometric wedge from which determinations are to be made, over the sample field of the densitometer. The metal tab 37 of the gamma scale bearing movable sector 36 is then held against the index arm 33 and both are moved together until the spot matches its eld, at which density is read from the hairline position on the Density scale. In the event the density selected is not between 0.8 and 1.0 the sensitometric wedge strip may be readjusted until a step is located of density between 0.8 and 1.0, or as close as possible to 0.8. The strip is then moved two steps in the direction of increasing density. The sector 36 remains in place as the plastic arm 33 is again moved until the spot matches its field,

. at which position the value of gamma is read from the position of the hairline 34 on the gamma scale. Speed is determined by placing a clear area of a sensitometric strip over the sample field of the densitometer and zero setting the instrument as before. Step 9 of the sensitometric strip is then placed over the sample field and the index arm is moved until the spot matches its field, when the speed of the strip on the step 9 speed scale is read from the hairline. In a similar manner sensitometric strip steps, other than step '9, are placed over the instrument sample field and speed readings are taken from the resultant positions of the hairline on the speed step scales.

Paper speed is the step number which has a transmission density of 1.65. In the absence of a step of the density 1.65, then adjacent density values are interpolated.

The densitometer disclosed herein has a limited range density scale which may be used for measuring the densities of negatives and the like. In making a density reading on a negative the instrument is zero set as before. the negative is mounted in the sample field and the indicating arm is moved until the spot matches its field. The vdensity of the negative is then read from the hairline on the density scale.

It will be apparent that movements of the indicator arm 33 moves the block 30 and the angle at which light from the lamp 16 passes through the block before it is incident on the upper surface of the angularly mounted mirror 12. The light incident to the upper face of the mirror l1?. provides the field observed at the peephole 10. Light passing through the hole 21 in the mirror 12 from the lamp 17 is the observed spot.

lt is to be understood that the device which is illustrated and described herein is made up of parts which inay be modilied somewhat without departing from the scope of the present invention.

I claim:

A visual densitometer having a peephole type of optical system brought to adjustment by matching light discs and read directly from surface scales at the light disc matched position comprising an upper compartment with a front panel and a top, a lower compartment, and a hinge joining the two compartments along a pair of adjacent edges t'hereof such that the upper compartment may be moved in an arc around the hinge as center of rotation; a pair of spring brass clips secured at one end adjacent the hinge and unsecured remote from the hinge for the removable insertion thereunder of and the releasable retention thereby of a sample strip of an optical density to be determined and the clips holding the sample strip against the upper surface of the lower compartment; a pair of optically aligned opal glasses respectively in the bottom of the upper compartment and the top of the lower compartment and providing a pair of sample windows between which the sample strip is releasably retained; the optical system comprising a peephole through the top of the upper compartment and coaxially aligned with the center of the pair of sample windows, a rst light source in the lower compartment below and directly visible through the pair of sample windows, a lens in the upper compartment and below the peephole for enlarging the pattern image of the sarnple between the pair of sample windows, a centrally apertured silvered mirror in the upper compartment and disposed between the lens and the pair of sample windows and with its aperture aligned in the optical system between the peephole and the first light source and with the plane of the mirror inclined at an angle to the line between the peephole and the iirst light source, a second light source in the upper compartment and spaced from the apertured and inclined mirror and applying light energy thereto with its reflection through the lens visible as a peripheral 4light around the image of the first light source defined by the aperture in the inclined mirror, a translucent block progressively insertable in the path of light from the second light source in matching the appearance through the peephole of the peripheral ring reiiected by the apertured mirror from the second light source with the center light transmitted through the center of the apertured mirror from the first light source as both the reflected light and the transmitted light are modified by the optical density of the sample strip between the pair of sample windows as releasably maintained in place by the pair of spring brass clips, a bearing shaft journalled in the front panel of the upper compartment and to which shaft one end of the said translucent block is attached for rotary motion around the shaft such that rotary motion of the shaft introduces at a progressively changing angle the translucent block into the light beam from the second light source to the peripheral ring of the apertured mirror and the shaft projecting through the front panel of the upper compartment for the adjustment of the translucent block by operation of the shaft from outside the upper compartment, and an index arm mounted at one of its ends on the shaft to be moved parallel to the front panel outside of the upper compartment and the index arm being provided with a hairline for sweeping a first outer graduated fixed density scale and calibrated along the arc described by the end of the index arm remote from its mount on the shaft and the index arm sweeping a second inner graduated gamma scale on a movable sector pivoted at one end on the shaft and the movable sector being provided with a sector ear engageable by the index arm and for use in adjusting the position of the sector preparatory to taking readings from the densitometer.

References Cited in the le of this patent UNITED STATES PATENTS 1,669,659 Godley May 15, 1928 2,043,908 Tuttle June 9, 1936 2,146,904 McFarlane et al. Feb. 14, 1939 2,235,590 Rockwell Mar. 18, 1941 2,359,873 Poindexter Oct. 10, 1944 OTHER REFERENCES Dudley: A Densitometer Simplified and Inexpensive, Photo Technique, I une 1940, pages 52-54.

`Coote: The Construction of a Transmission Densitometer, The British Journal of Photography, February 21, 1941, pages 87-89. 

